The loss of the traditional reactor made of silicon steel sheet increases sharply due to the switching frequency up to thousands of Hertz in the current application fields of high-power frequency converter, UPS (Uninterruptible Power Supply) and new energy, which causes that the traditional reactor made of silicon steel sheet cannot adapt to the high-frequency application fields. Thus, the alloy powder block core, Amorphous and Nanocrystalline are usually used in the reactor of high power and high-frequency, and the JFE Corporation of Japan uses the super silicon steel with the silicon content 6.5% deposited by chemical vapor infiltration in recent years, which is a good choice.
The magnetic core (iron core) made of non-crystalline material usually can be made by laminating strips, and the super silicon steel is also made by stacking sheet materials. As same as copper foils and aluminum foils, the magnetic core and the super silicon steel both are continuous flat conductor or curved conductor, which causes a huge eddy current loss once there is an alternating magnetic flux in the same or similar direction with the normal direction of the flat surface or the curved surface of the conductor.
In accordance with the relationship among magnetic flux, resistance and magnetic motive force in magnetic circuit, the distributions of the magnetic motive force in a magnetic circuit are in direct proportion to the resistance of this magnetic circuit. Generally, the calculation formula of the magnetic motive force is as follows:NI=Φ·R1+ . . . +Φ·Rn=Φ·le1/(μe1·Ae1)+ . . . +Φ·len/(μen·Aen)
Where, NI indicates the magnetic motive force; Φ indicates the magnetic flux; R indicates the resistance; l indicates the length of the magnetic circuit; μ indicates the relative permeability of the magnetic core; A indicates the sectional area of the magnetic core.
The common magnetic cores are all tangible solids. Being affected by visual factors, designers often only consider the solid magnetic core itself and the air gap connected in series with the solid magnetic core, but ignore that the whole intangible space is actually a magnetic path when they design a magnetic circuit. These intangible magnetic circuits are connected in series or in parallel with the solid magnetic cores, and have a great influence on the performance of the whole magnetic circuits. As the relative permeability of the space is very low (only “1” in value), in the space slightly farther away from the excitation source (e.g., windings), the magnetic field intensity with frequency less than the RF frequency would decay rapidly to a very low value that could be ignored. In the space near the excitation source, losses would be generated as long as the magnetic fields which are called Near Field Radiations meet a conductor.
Currently, in the alloy powder core reactor applied in the case where switching frequency is more than thousands of Hertz, usually, a normative square closed magnetic circuit is formed by stacking a plurality of the alloy powder block cores, as illustrated in FIG. 1. The stacked alloy powder block core 1 shown in FIG. 1 includes horizontal magnetic cores 1-1 and vertical magnetic cores 1-2. The reference sign 2 denotes windings (e.g., windings made of copper foils or aluminum foils) wound around the vertical magnetic cores 1-2 (i.e., core columns), while there is no windings wound around the horizontal magnetic cores 1-1 (i.e., yokes). Similar to an annular alloy powder core, the stacked alloy powder block core has a magnetic circuit with uniform resistance, and the difference between them is that the windings of the reactor made of the annular alloy powder core can be distributed uniformly along the perimeter of the core column. Thus, the magnetic motive force generated by the windings of the reactor made of the annular alloy powder core is distributed uniformly along the magnetic circuit of the core column, and the magnetic motive force can be consumed exactly by the uniform resistance of the core column, so the magnetic motive force would not be concentrated on part of the magnetic circuit. But for the alloy powder core formed by stacking, just like the stacked alloy powder block core 1 shown in FIG. 1, the windings only can be wound around two parallel columns, and no windings are wound around the other two columns (e.g., the horizontal magnetic core 1-1 shown in FIG. 1), which causes that the magnetic motive force generated by such windings cannot be distributed uniformly along the magnetic circuit and a serious near field radiation will be generated by the diffusion of magnetic flux due to local magnetic motive force concentration.
In FIG. 1, the magnetic motive force between the two terminals of the upper yoke and the lower yoke is NI·b/(2a+2b), where a is the horizontal side length of the rectangular magnetic circuit shown by the dotted line in FIG. 1, and b is the vertical side length of the rectangular magnetic circuit. Losses are generated when the radiated magnetic fluxes (e.g., the magnetic field lines shown by reference signs 3, 4, 5, 6 in FIG. 1) meet a conductor, and the losses are particularly serious when the direction of the magnetic flux is consistent with or close to the normal direction of the flat surface or the curved surface of the conductor. As the direction of magnetic field lines 4 and 5 in FIG. 1 is close to or consistent with the normal direction of the windings 2, serious eddy current losses will be generated on the windings 2 due to these near field magnetic fluxes.
Besides, the three-phase reactor is usually used in the current application fields of high-power frequency converter, UPS and new energy. The material of the yokes of an integrated three-phase reactor (for example, three-phase three-column reactor or three-phase five-column reactor) must have a very high relative permeability, otherwise, the electric inductances of the three phases will be in imbalance. The powder core material usually has a relative permeability which is not high, so the integrated three-phase reactor cannot be made of only the alloy powder core. And for the same electrical properties, the total volume of three single-phase reactors is larger than that of one three-phase reactor, thus, three single-phase reactors cannot be used as a substitution for one three-phase reactor in the situation where there is a requirement for the size of the reactor.
When the reactor for three-phase electricity is made of a material with high permeability, such as silicon steel sheet, amorphous nano-crystalline material, the three-phase three-column reactor (or three-phase five-column reactor) can be made because of the symmetry of three-phase electricity. The yoke of such reactor is an entirety without any air gap, and any additional loss will not be generated inside the yoke under such magnetic flux distribution. However, the air gap in the core column is necessary for avoiding the magnetic saturation of the core column. Because the relative permeability of silicon steel sheet is considerably larger than that of the air, the magnetic fluxes at the interface between an iron core and the air flow vertically in and out of the iron core.
For example, FIG. 2 illustrates a reactor, in which the core columns are made of a material with high relative permeability, the core columns are made by stacking laminated magnetic cores, and there are air gaps in the core column. The magnetic fluxes 10 and 20 shown in FIG. 2 are positioned in the magnetic cores. The magnetic core plane of which the magnetic fluxes 20 flow in and out is composed by stacking multiple laminated magnetic cores insulated from each other and high eddy current will not be generated within the plane; but the magnetic core plane of which the magnetic fluxes 10 flow in and out is an entirety and serious additional eddy current losses are generated due to the huge eddy current (as shown by reference signs 30 and 40 in FIG. 2) induced within the plane, and the diffused magnetic fluxes will have a great influence on the losses of nearby conductors (e.g., windings, components, etc.).
To overcome the above drawbacks, two core column materials with different relative permeability need to be combined, so as to maybe eliminate the magnetic flux which is consistent with the normal direction of the planar conductor, so that the eddy current losses may be reduced significantly.